Master/slave frequency locked loop

ABSTRACT

A master/slave configuration of a frequency locked Loop (FLL) decouples the process, target voltage, temperature (PVT) tracking goals of locking the loop from adapting the clock frequency in response to voltage droops in the supply. A master oscillator circuit receives a regulated supply voltage and supplies a master oscillator signal. A control circuit supplies a master frequency control signal to control a frequency of the master oscillator signal to a target frequency. A slave oscillator circuit is coupled to a regulated supply voltage and a droopy supply voltage and supplies a slave oscillator signal having a frequency responsive to a slave frequency control signal that is based on the master frequency control signal. The frequency of the second oscillator signal is further responsive to a voltage change of the droopy supply voltage.

BACKGROUND Description of the Related Art

Integrated circuits utilize clock signals to synchronize the flow ofdata through the integrated circuit. The data flow can be subject tostrict timing requirements and the timing of the clock signals can varywith variations in the supply voltage. Noise affects power supplies inintegrated circuits and can cause the voltage to vary above (overshoot)or below (droop) a nominal voltage level. Noise can adversely affecttiming margins associated with the clock signals. Noise may be caused bydeterministic noise sources and random noise sources. Present day highspeed integrated circuits such as graphic processor units (GPUs) orcentral processing units (CPUs) compensate for noise on the power supplyline by lowering the clock frequency in response to a voltage droop toallow more time for data signals to get from their source to theirdestination. Absent such adaptive clocking approaches that provide extratiming margin, failures can occur as a result of voltage droop.

Conventional adaptive clocking approaches utilize delay locked loops(DLLs) or frequency locked loops (FLLs) to lock frequency of the clocksignal supplied to the integrated circuit components with the systemclock being generated. That causes circuits in the adaptive clock system(using the DLL or FLL) to lock to an average noisy voltage and averageclock frequency, causing uncertainty due to dependence on the powerdelivery response and application characteristics. In particular, theadaptive clock system can lock to low frequency noise that can bepresent due to temperature variations or low frequency voltagevariations.

SUMMARY OF EMBODIMENTS OF THE INVENTION

Accordingly, embodiments herein utilize a master/slave configuration ofa Frequency Locked Loop (FLL) to decouple the process, target voltage,temperature (PVT) tracking goals of locking the loop from adapting theclock frequency in response to voltage droops or overshoots in thesupply voltage.

Accordingly, in one embodiment an apparatus includes a first oscillatorcircuit coupled to a first supply voltage and configured to supply afirst oscillator signal. The first supply voltage is a regulated supplyvoltage. A control circuit is coupled to receive the first oscillatorsignal and to receive an indication of a target frequency and isconfigured to supply a first frequency control signal to control afrequency of the first oscillator signal to the target frequency. Asecond oscillator circuit is configured to supply a second oscillatorsignal having a frequency responsive to a second frequency controlsignal. The second frequency control signal is based on the firstfrequency control signal. The frequency of the second oscillator signalis further responsive to a voltage change of the second supply voltage.

In another embodiment, a method for compensating a clock signal in anintegrated circuit includes supplying a first oscillator signal from afirst oscillator circuit that receives a first supply voltage. Themethod further includes receiving an indication of a target frequency ata control circuit and generating a first frequency control signal tocontrol a frequency of the first oscillator signal based on the targetfrequency and the frequency of the first oscillator signal. The methodfurther includes supplying a second frequency control signal to a secondoscillator circuit receiving the first supply voltage and receiving asecond supply voltage. The second frequency control signal is based onthe first control signal. The second oscillator circuit supplies asecond oscillator signal with a frequency of the second oscillatorsignal based on the second frequency control signal. The secondoscillator circuit also adjusts the frequency of the second oscillatorsignal responsive to a voltage change associated with the second supplyvoltage.

In another embodiment an apparatus includes a master oscillator circuitcoupled to a regulated supply voltage. The master oscillator circuit isconfigured to supply a master oscillator signal. A control circuit iscoupled to the master oscillator signal and is configured to supply amaster frequency control signal to control a frequency of the masteroscillator signal to a target frequency. A slave oscillator circuit iscoupled to a regulated supply voltage and a droopy supply voltage and isconfigured to supply a slave oscillator signal having a frequencyresponsive to a slave frequency control signal that is based on themaster frequency control signal. The frequency of the second oscillatorsignal is further responsive to a voltage change of the droopy supplyvoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates an exemplary system including a master oscillator anda slave oscillator.

FIG. 2 illustrates a high level block diagram of an embodiment of aslave oscillator.

FIG. 3 illustrates a timing diagram illustrating operation of theC-element of the slave oscillator.

FIG. 4 illustrates a high level block diagram showing additional detailsof an embodiment of a slave oscillator.

FIG. 5 illustrates a high level block diagram of an embodiment in whicha master oscillator controls multiple slave oscillators.

FIG. 6 illustrates an embodiment of a control structure to generateoffsets for the slave oscillator.

FIG. 7 illustrates how the effective frequency can be tracked andadjusted by a control loop to be closer to a target frequency.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

Referring to FIG. 1, an exemplary system includes a master oscillator101 and a slave oscillator 103. In the illustrated embodiments, both themaster and slave oscillators are digitally controlled oscillators(DCOs). A frequency-locked loop (FLL) that includes the masteroscillator 101 and the loop control logic 104 tracks frequency changescaused by voltage and temperature changes associated with the masteroscillator 101. The loop control logic 104 supplies the masteroscillator with a frequency control word (FCWM) 105. The masteroscillator 101 receives a regulated voltage 112 from regulator 107. Thatensures that the voltage received by the master DCO 101 is much“cleaner” than the noisy or droopy VDD voltage 111 received by the slaveoscillator 103. The droopy voltage may be, e.g., the voltage utilized bya processor core. The slave oscillator 103 utilizes the maser oscillatorcontrol signal to control frequency and also adapts the slave oscillatoroutput 109 to variations in the droopy voltage 111 as explained furtherherein.

The slave oscillator 103 receives a slave frequency control word (FCWS)115 based on the master frequency control word FCWM from the summingcircuit 114. That allows an offset to be added to the FCWM to create theslave frequency control word FCWS 115. The offset can have a value ofzero in which case the slave oscillator receives the FCWM unchanged. Inembodiments, the slave and master oscillators are located in closephysical proximity so PVT variations that affect the master oscillatoralso affect the slave oscillator. While the master and slave oscillatorsmay be digitally controlled oscillators as shown in the embodiment ofFIG. 1, in other embodiments the oscillators may be voltage or currentcontrolled oscillators. The control signal FCWM and FCWS may bemulti-bit digital signals to control the frequency of the DCOs. In otherembodiments the FCWM and FCWS are voltage or current signals for analogcontrol of the oscillators. In analog embodiments the summer circuit 114is implemented to add or subtract voltages or currents.

Loop control logic 104 receives a reference clock 121 having a knownfrequency. In the illustrated embodiment the reference clock may bedivided down in divider 123. A counter 125 counts a predetermined numberof reference clock periods to provide a sampling window for the masteroscillator clock signal 127. Counter 129 counts the master oscillatorclock signal (or a divided down version) during the sampling window. Inthe illustrated embodiment, a divider 131 divides the master oscillatorclock signal 127. A proportional and integral (PI) controller 133receives the count of the number of divided down master oscillator clocksignal periods over the sampling window and compares the sampled countvalue to a target count value 135. The target count value represents thetarget frequency for the master oscillator clock signal 127. While theillustrated embodiment utilizes a PI controller 133, other controlapproaches may be utilized in other embodiments. The control logic 104may be implemented in a microcontroller or other control logic. Thecontrol loop tracks low frequency changes resulting from changes involtage and temperature and also compensates for process variationsassociated with the particular die, which allows the slave oscillator103 to be isolated from low frequency PVT variations and respond to thehigher frequency voltage droops or overshoots that occur in the noisyvoltage supplied to a voltage domain of an integrated circuit.

While FIG. 1 shows separate regulator blocks 107 and 108, embodimentsmay utilize a single regulator to supply the regulated voltage to themaster and slave oscillators 101 and 103 in which case the regulatedvoltages 112 and 113 are the same voltage. A selector circuit 141receives the master oscillator output signal 127 and the slaveoscillator output signal 109. The selector circuit may select the masteroscillator signal to be output from selector circuit 141 for testpurposes. If the adaptive clock generator is on a voltage supply that isshut off and the clock is still needed by other logic, the clock signalfrom the master oscillator can be selected operationally.

FIG. 2 illustrates a high level block diagram of an exemplary slaveoscillator 103 that functions as an adaptive oscillator to slow downsystem clocks in response to a voltage droop and to limit the frequencyresponse to voltage overshoots. The adaptive slave oscillator 103includes a reference delay line 201 that operates with the regulatedvoltage VDD 113 and a droopy delay line 203 that operates with thenoisy/droopy voltage 111. The voltage supplied to delay line 201 is“clean” and the clock signal A should be more stable than clock signalB. The delay line 203 outputs a clock signal B that varies in frequencywith the droopy voltage. When the droopy voltage 111 droops, the delayline slows down and when the droopy voltage increases, the delay linespeeds up. Note that the inverter 207 forms the fifth inverter to ensureeach of the delay lines 201 and 203 functions as an oscillator.

As shown in FIG. 3, the C-element logic 205 outputs an output signal Cwith a logic high when both inputs A and B are high at 301. TheC-element logic 205 keeps the output signal C high until both inputs Aand B are low at 303. Once the output C is a logic low, the C-elementmaintains the output C low until both inputs again become high at 305.As shown in FIG. 3 when clock signal A leads clock signal B, clocksignal C is the same as clock signal B. When clock signal B leads clocksignal A, clock signal C is the same as clock signal A. Thus, theC-element logic 205 outputs a logic high output signal C when bothinputs A and B are high at 307 and keeps the output C high until bothinputs become low at 309. Once the output C is a logic low, theC-element maintains the output C low until both inputs again become highat 311. In other words, the logic 205 selects the latest pulse tooutput.

FIG. 4 shows a more detailed view of an embodiment of the adaptive slaveoscillator 103. The embodiment includes a reference delay line 401 thatreceives a regulated voltage 402 and a droopy delay line 403 thatreceives a droopy voltage 404. Each of the delay lines has a number ofrows of delay elements that can be turned on or off. The more rows thatare turned on, the faster the delay element operates. In an embodimentthe delay lines 401 and 403 have 256 rows. Other embodiments can haveother numbers of rows. Embodiments may use tristate inverters shown inFIG. 4, buffers, or other forms of delay elements. In addition, whilesingle-ended embodiments are illustrated, other embodiments may utilizedifferential delay lines.

A frequency control word (FCW) 405 selects how many rows in each of thedelay lines are enabled and therefore the output frequency of each ofthe delay lines. In the illustrated embodiment a bias setting 407controls the variable capacitors in each of the delay lines. Inaddition, an offset value 409 can be used to adjust the FCW 405 suppliedto the reference delay line 401 if desired. The droopy delay line 403receives an offset value 411 that can be used to adjust the FCW 405supplied to the droopy delay line 403. The offset logic for thereference delay line and the droopy delay line is not shown for ease ofillustration. The embodiment illustrated in FIG. 4 effectively dealswith high frequency noise such as voltage droop but does not work wellfor low frequency noise. In fact, the circuit of FIG. 4 may lock ontolow frequency noise and fail to compensate for the noise. Utilizing themaster oscillator control loop (see FIG. 1) more effectively compensatesfor low frequency noise in the system.

In an embodiment, the master oscillator 101 (FIG. 1) includes two delaylines such as shown in FIG. 4. However, rather than one of the delaylines receiving a regulated voltage supply and one of the delay linesreceiving a droopy voltage supply, each of the delay lines receives theregulated voltage supply. Thus, in such an embodiment, the C-elementoutput reflects both the delay lines of the master oscillator.

Referring back to FIG. 1 the summer circuit 114 forms the slave controlword FCWS 115 from the master frequency control word FCWM and an offsetvalue 136 generated in offset calculation logic 137. The offsetcalculation logic receives inputs 139 and 140 that cause the offsetcalculation logic to increase or decrease the offset value 136 suppliedto summer circuit 114 to thereby increase or decrease the FCWM 105before it is supplied to the slave oscillator 103. The offset value 136may be zero in which case the slave oscillator uses the master frequencycontrol word 105 without alteration. In certain situations, e.g., adetected current excursion, e.g., a current overshoot or undershoot,offset calculation logic 137 receives a force signal 139 to adjust thefrequency supplied by the slave oscillator by adding or subtracting anoffset from the master frequency control word depending on the directionof the current excursion. Other situations may result in a nonzerooffset. For example, two independent clock domains may be operating inthe integrated circuit and the domain supplied by the slave DCO 103 mayneed to slow down in order to avoid overrunning a FIFO in the otherclock domain. In addition, the offset may be used to effect variouspower management outcomes to speed up or reduce the clock frequencysupplied by the slave oscillator. In the illustrated embodiment, theoffset calculation logic 137 receives the master frequency control word105 in order to help determine an appropriate offset amount. While shownseparately, the summer 114 and offset calculation logic 137 may becombined and supply the modified FCWM as the FCWS. In an embodiment, theslave oscillator 103 has two offset calculation blocks 137 (only oneshown) and two summing circuits 114 (only one shown). One of the offsetcalculation blocks and summing circuit is for the reference delay line(e.g., 201, 401 in FIGS. 2 and 4) and the other offset calculation blockand summing circuit is for the droopy delay line (see, e.g., 203, 403 inFIGS. 2 and 4). That allows for independent adjustment of the frequencyof the slave reference and droopy delay lines that are otherwisecontrolled by the master frequency control word.

FIG. 5 illustrates an embodiment in which master oscillator 501 controlstwo slave oscillators 503 and 505. Slave oscillator 503 receives themaster frequency control word (FCWM) through a summer circuit 507 andslave oscillator 505 receives the master frequency control word (FCWM)through a summer circuit 509. Slave oscillator 503 receives droopyvoltage VDD(0) and slave oscillator 505 receives droopy voltage VDD(1).VDD(0) and VDD(1) can be derived from a common input voltage rail bututilized in separately controlled voltage and clock domains. Forexample, one voltage domain may be turned off while the other voltagedomain remains powered. The two slave oscillators may receive differentreference voltages. Separate offset calculation blocks 511 and 515independently determine any required offsets for the two slaveoscillators frequency control words FCWS0 and FCWS1.

FIGS. 6 and 7 provide an example of how the Stretch Amount may bedetermined and supplied to the offset calculation logic 137 (FIG. 1). Inaddition to the control loop 104, the clock logic may include logic todetermine an effective frequency of the slave oscillator over apredetermined time period. The slave oscillator output signal 601 and aknown reference clock signal 603 are supplied to an effective frequencycalculation logic 605. The effective frequency calculation logic countsthe number of slave oscillator clock cycles that occur over apredetermined number of reference clock cycles in order to determine aneffective frequency. The number of reference clock cycles corresponds toa particular time period, e.g., 10 ms. The microcontroller 607 mayoperate a proportional integral derivative (PID), a PI control loop, oranother form of control to control the effective frequency.

Referring to FIG. 7, the control logic sets a maximum frequency Fmax701. Curve 703 represents the actual frequency. As can be seen theactual frequency is limited by the maximum frequency. The controller mayensure that the frequency is limited to the maximum frequency using thestretch amount input 140 to the offset calculation logic 137 to limitthe frequency (see FIG. 1) by reducing the frequency control word andslowing down the slave oscillator. The target frequency is shown as 705.If over time the effective frequency 707 is too far below the targetfrequency 705, the stretch amount input 140 can be used to increase thefrequency of the slave oscillator and over time increase the effectivefrequency to be closer to the target frequency as shown in FIG. 7. Whenthe effective frequency is above the target frequency, the effectivefrequency can be reduced using the offset calculation logic to adjustthe master frequency control word to reduce the slave oscillatorfrequency. In addition to controlling frequency through the offsetapplied to the master frequency control word, the controller can alsoincrease or decrease VDD in response to the effective frequency beingabove or below a target value.

While circuits and physical structures are generally presumed for someembodiments, it is well recognized that in modern semiconductor designand fabrication, physical structures and circuits may be embodied incomputer-readable descriptive form suitable for use in subsequentdesign, test or fabrication stages. Structures and functionalitypresented as discrete components in the exemplary configurations may beimplemented as a combined structure or component. Embodiments arecontemplated to include circuits, systems of circuits, related methods,and non-transitory computer-readable medium encodings of such circuits,systems, and methods, all as described herein, and as defined in theappended claims. As used herein, a non-transitory computer-readablemedium includes at least disk, tape, or other magnetic, optical,semiconductor (e.g., flash memory cards, ROM), or electronic medium.

Thus, embodiments have been described that decouple the tracking goalsfor target voltage and temperature from adapting the clock frequency inresponse to high frequency voltage variations in the supply voltage. Thedescription set forth herein is illustrative, and is not intended tolimit the scope of the following claims. Other variations andmodifications of the embodiments disclosed herein, may be made based onthe description set forth herein, without departing from the scope setforth in the following claims.

What is claimed is:
 1. An apparatus comprising: a first oscillatorcircuit coupled to a first supply voltage and configured to supply afirst oscillator signal, the first supply voltage being a regulatedsupply voltage; a control circuit coupled to the first oscillator signaland coupled to an indication of a target frequency and configured tosupply a first frequency control signal to control a frequency of thefirst oscillator signal to the target frequency; a second oscillatorcircuit configured to supply a second oscillator signal having afrequency responsive to a second frequency control signal, the secondfrequency control signal being based on the first frequency controlsignal and wherein the frequency of the second oscillator signal isfurther responsive to a voltage change of a second supply voltage; and acircuit to combine the first frequency control signal and an offset togenerate the second frequency control signal.
 2. The apparatus asrecited in claim 1, wherein the second oscillator circuit furthercomprises: a first delay line coupled to the first supply voltage oranother regulated supply voltage and coupled to the second frequencycontrol signal, wherein the first delay line supplies a first delay lineoutput signal; a second delay line coupled to the second supply voltageand coupled to the second frequency control signal, wherein the seconddelay line supplies a second delay line output signal; and a logiccircuit to supply the second oscillator signal according to a logicalrelationship of the first delay line output signal and the second delayline output signal.
 3. The apparatus as recited in claim 2, wherein thefirst oscillator circuit further comprises: a third delay line coupledto the regulated supply voltage and coupled to the first frequencycontrol signal, wherein the third delay line supplies a third delay lineoutput signal; a fourth delay line coupled to the regulated supplyvoltage and coupled to the first frequency control signal, wherein thefourth delay line supplies a fourth delay line output signal; and asecond logic circuit to supply the first oscillator signal according toa logical relationship of the third delay line output signal and thefourth delay line output signal.
 4. The apparatus as recited in claim 1,wherein the first frequency control signal is a first digital signal andthe offset is a second digital signal and the circuit is a summingcircuit to add or subtract the second digital signal to or from thefirst digital signal.
 5. The apparatus as recited in claim 1, whereinthe first oscillator circuit tracks variations in temperature andvoltage variations in the regulated supply voltage.
 6. The apparatus asrecited in claim 1, further comprising a selector circuit to selectbetween the first oscillator signal and the second oscillator signal. 7.The apparatus as recited in claim 1, further comprising: a thirdoscillator circuit coupled to a third supply voltage and coupled to athird frequency control signal and configured to supply a thirdoscillator signal having a third frequency responsive to variations ofthe third supply voltage; and wherein the frequency of the thirdoscillator signal is further responsive to the third frequency controlsignal, the third frequency control signal being based on the firstfrequency control signal.
 8. The apparatus as recited in claim 7,further comprising: a second circuit to combine the first frequencycontrol signal and an offset and supply the third frequency controlsignal.
 9. A method for compensating a clock signal in an integratedcircuit comprising: supplying a first oscillator signal from a firstoscillator circuit receiving a first supply voltage; receiving anindication of a target frequency at a control circuit; generating afirst frequency control signal to control a frequency of the firstoscillator signal based on the target frequency and the frequency of thefirst oscillator signal; supplying a second frequency control signal toa second oscillator circuit receiving the first supply voltage andreceiving a second supply voltage, the second frequency control signalbased on the first frequency control signal; supplying a secondoscillator signal from the second oscillator circuit with a frequency ofthe second oscillator signal based, at least in part, on the secondfrequency control signal; and adjusting the frequency of the secondoscillator signal responsive to a voltage change associated with thesecond supply voltage.
 10. The method as recited in claim 9, furthercomprising: supplying the second frequency control signal and the firstsupply voltage to a first delay line of the second oscillator circuit;supplying a first delay line output signal from the first delay line;supplying the second frequency control signal and the second supplyvoltage to a second delay line of the second oscillator circuit;supplying a second delay line output signal from the second delay line;supplying the first delay line output signal and the second delay lineoutput signal to logic circuit; and supplying the second oscillatorsignal from the logic circuit according to a logical relationship of thefirst delay line output signal and the second delay line output signal.11. The method as recited in claim 9, wherein the first supply voltageis a regulated supply voltage and wherein the second supply voltage is adroopy supply voltage.
 12. The method as recited in claim 9, furthercomprising: combining the first frequency control signal and an offsetsignal to generate the second frequency control signal.
 13. The methodrecited in claim 9, further comprising the second oscillator circuittracking variations in voltage of the second supply voltage to generatethe second oscillator signal.
 14. The method recited in claim 9, furthercomprising the first oscillator circuit tracking variations intemperature and variations in voltage of the first supply voltage. 15.The method recited in claim 9, further comprising selecting in aselector circuit one of the first oscillator signal and the secondoscillator signal.
 16. The method recited in claim 9, furthercomprising: supplying a third supply voltage and the first supplyvoltage to a third oscillator circuit; supplying a third frequencycontrol signal to the third oscillator circuit to control, at least inpart, a frequency of a third oscillator signal; and adjusting thefrequency of the third oscillator signal responsive to variations of thethird supply voltage.
 17. The method recited in claim 16, furthercomprising: combining the first frequency control signal and a secondoffset signal to generate the third frequency control signal.
 18. Anapparatus comprising: a master oscillator circuit coupled to a regulatedsupply voltage and configured to supply a master oscillator signal; acontrol circuit coupled to the master oscillator signal configured tosupply a master frequency control signal to control a frequency of themaster oscillator signal to a target frequency; and a slave oscillatorcircuit coupled to a second regulated supply voltage and a droopy supplyvoltage and configured to supply a slave oscillator signal having afrequency responsive to a slave frequency control signal based on themaster frequency control signal and wherein the frequency of the slaveoscillator signal is further responsive to a voltage change of thedroopy supply voltage.
 19. The apparatus as recited in claim 18, furthercomprising: an arithmetic circuit to modify the master frequency controlsignal according to an offset signal and supply the slave frequencycontrol signal.
 20. The apparatus as recited in claim 18 where theregulated supply voltage and the second regulated supply voltage are thesame voltage.